What is NPSH in Pumps

NPSH stands for Net Positive Suction Head and is a measure of the pressure experienced by a fluid on the suction side of a centrifugal pump.

The difference between inlet pressure and the lowest pressure level inside the pump is called NPSH. NPSH is therefore an expression of the pressure loss that takes place inside the first part of the pump housing. 

Pump designers use NPSH to ensure that pumps will operate without internal damage caused by cavitation under all specified operating conditions.

Introduction

• A Centrifugal pump is shown in the figure, with suction pipe connected to a sump. The pressure at sump is atmospheric pressure P_a since it is open to atmosphere.
• Now for fluid to be sucked in the pump, the pressure at impeller eye (pump inlet) should be less than atmospheric pressure, so due to pressure difference fluid will be sucked in. So we it can be written as

• The Inlet pressure (Suction head h_s )should not be such less that it can reach the vapor pressure of the fluid.
• Vapour pressure (P_v) is a pressure at which liquid start converting into water vapour.
• This water vapour forms air bubble which may expands & collapse at the surface of the pipe & leads to damage.

• This phenomenon is known as Cavitation.

Low pressure at the suction side of a pump may cause the fluid to start boiling with

• reduced efficiency
• cavitation
• damage

Boiling starts when the pressure in the liquid is reduced to the vapor pressure of the fluid at the actual temperature.

• Hence in order to avoid cavitation Inlet suction pressure ( h_s ) should be higher than Vapour pressure(P_v). The equation can be written as

This is the basic condition we require in order to save the Centrifugal pump from damage.

Derivation of NPSH

In order to find NPSH, Applying Bernoulli’s equation at point 1 & 2 in fig 1.

Now,

• V₁ =0 …. Fluid is stationary at pt 1
• Z₁=0   ….. Datum
• h_f = frictional loss
• P₁=P_a we get,

This equation satisfies 1^st condition mentioned above i.e pressure at point 2 should be less than atmospheric pressure.

Now to avoid cavitation,

Hence,

Fluid at impeller eye have some velocity also, so

This is the final value of NPSH

Available NPSH – NPSH_a or NPSHA

The Net Positive Suction Head available from the application to the suction side of a pump is often named NPSH_a. The NPSH_a can be estimated during the design and the construction of the system, or determined experimentally by testing the actual physical system.

Required NPSH – NPSH_r or NPSHR

The NPSH_r, called as the Net Suction Head as required by the pump in order to prevent cavitation for safe and reliable operation of the pump.

The required NPSH_r for a particular pump is in general determined experimentally by the pump manufacturer and a part of the documentation of the pump.